Biodiesel has been the subject of much investigation as an alternative for petroleum diesel fuel. As used herein, the term “biodiesel” refers to an ester-based fuel oxygenate that is derived from a biological source. The biodiesel is used as an alternative for, or as an additive to, petroleum diesel fuel in automobiles or other vehicles. The biodiesel is typically produced from a triglyceride starting material or a fatty acid starting material by a transesterification reaction or an esterification reaction, respectively. Generally, the triglyceride is reacted, or transesterified, with an alcohol to produce glycerol (also known as glycerin) and a corresponding alkyl ester of the triglyceride. Similarly, the fatty acid is reacted, or esterified, with an alcohol to produce a corresponding alkyl ester of the fatty acid. The triglyceride or fatty acid starting materials are available from various sources, such as from pure or used fats or oils. These fats or oils are typically hydrophobic and water-insoluble. In addition to including the triglyceride or fatty acid, the fats or oils include free fatty acids, phospholipids, sterols, water, dirt, detergents, polar compounds, or other impurities. These sources of the fats or oils are too viscous to be used directly as the biodiesel fuel and, therefore, the triglycerides or fatty acids are transesterified or esterified to produce the corresponding alkyl ester, which has a lower viscosity than that of the source of the starting material. The transesterification of the triglyceride (or the esterification of the fatty acid) is conducted with an excess of the alcohol in the presence of a catalyst. As the reaction proceeds, two phases form. One phase includes the alkyl ester and the other phase includes the glycerol. The two phases are allowed sufficient time to settle before additional processing is conducted to purify the alkyl ester from the glycerol.
To produce high quality biodiesel, the source of the triglyceride or fatty acid starting material should include a minimal amount of water. The water, if present, reacts with the free fatty acids and the catalyst to form soap. Water also inhibits the equilibrium of the transesterification or esterification reaction. The soap also increases the viscosity of the alkyl ester, decreases the yield of the alkyl ester, decreases the quality of the alkyl ester, decreases the ability to separate the alkyl ester from the glycerol, consumes the catalyst, decreases the efficiency of the catalyst, and forms gels. The triglyceride or fatty acid starting material should also include a minimal amount of other impurities, such as nonpolar impurities. If present, these nonpolar impurities reduce the effectiveness and stability of the catalyst or decrease the quality of the reaction products. While using pure or refined fats or oils as the starting material avoids these problems, the pure fats or oils are expensive and add considerable expense to the cost of producing the alkyl ester. Used fats or oils, such as those generated by households or the food service industry, are less expensive but typically contain higher amounts of water. Water is also produced as a byproduct of the esterification reaction or is introduced into a product stream by water washing of the alkyl ester. Since the water affects the quality of the alkyl ester, the water must be removed before using the alkyl ester for its intended purpose.
The water is typically removed by distillation, evaporization, lyophilization, use of an inert gas, or use of a drying agent, such as a molecular sieve. For instance, the feedstock stream or the product stream is heated above the boiling point of water to evaporate the water or is heated to distill and condense the water. However, since these techniques consume considerable amounts of energy or use expensive and toxic compounds, many of these techniques are not economical.
It would be desirable to remove the water and other polar impurities from the feedstock stream or the product stream to produce a feedstock stream or a product stream free from the impurities.